Connection between planetary He I λ10 830 Å absorption and extreme-ultraviolet emission of planet-host stars

¹ Centro de Astrobiología, CSIC-INTA, Camino bajo del Castillo s/n, 28692 Villanueva de la Cañada, Madrid, Spain
² Instituto de Astrofísica de Andalucía (IAA-CSIC), Glorieta de la Astronomía s/n, 18008 Granada, Spain
³ Thüringer Landessternwarte Tautenburg, Sternwarte 5, 07778 Tautenburg, Germany
⁴ Institut für Astrophysik und Geophysik, George-August-Universität, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany
⁵ Instituto de Astrofísica de Canarias (IAC), 38205 La Laguna, Tenerife, Spain
⁶ Departamento de Astrofísica, Universidad de La Laguna (ULL), 38206 La Laguna, Tenerife, Spain
⁷ Landessternwarte, Zentrum für Astronomie der Universität Heidelberg, Königstuhl 12, 69117 Heidelberg, Germany
⁸ Institut de Ciències de l’Espai (ICE, CSIC), Campus UAB, Can Magrans s/n, 08193 Bellaterra, Barcelona, Spain
⁹ Institut d’Estudis Espacials de Catalunya (IEEC), 08860 Castelldefels, Barcelona, Spain

^★ Corresponding author; jsanz@cab.inta-csic.es

Received: 27 July 2024
Accepted: 12 December 2024

Abstract

Context. The detection of the He I λ10 830 Å triplet in exoplanet atmospheres has opened a new window for probing planetary properties, including atmospheric escape. Unlike Lyman α, the triplet is significantly less affected by interstellar medium (ISM) absorption. Sufficient X-ray and extreme ultraviolet (XUV) stellar irradiation may trigger the formation of the He I triplet via photoionization and posterior recombination processes in the planet atmospheres. Only a weak trend between stellar XUV emission and the planetary He I strength has been observed so far.

Aims. We aim to confirm this mechanism for producing near-infrared He I absorption in exoplanetary atmospheres by examining a substantial sample of planetary systems.

Methods. We obtained homogeneous measurements of the planetary He I line equivalent width and consistently computed the stellar XUV ionizing irradiation. Our first step was to derive new coronal models for the planet-host stars. We used updated data from the X-exoplanets database, archival X-ray spectra of M-type stars (including AU Mic and Proxima Centauri), and new XMM-Newton X-ray data recently obtained for the CARMENES project. These data were complemented at longer wavelengths with publicly available HST, FUSE, and EUVE spectra. A total of 75 stars are carefully analyzed to obtain a new calibration between X-ray and extreme ultraviolet (EUV) emission.

Results. Two distinct relationships between stellar X-ray emission (5–100 Å) and EUV_H (100–920 Å) or EUV_He (100–504 Å) radiation are obtained to scale the emission from late-type (F to M) stellar coronae. A total of 48 systems with reported planetary He I λ 10 830 Å studies, including 21 positive detections and 27 upper limits, exhibit a robust relationship between the strength of the planetary He I feature and the ionizing XUV_He received by the planet, corrected by stellar and planetary radii, as well as the planet’s gravitational potential. Some outliers could be explained by a different atmospheric composition or the lack of planetary gaseous atmospheres. This relation may serve as a guide to predict the detectability of the He I λ 10 830 Å absorption in exoplanet atmospheres.